1. Technical Field
The present invention relates generally to semiconductor devices, and more particularly, to forming a varied impurity profile region for varying breakdown voltages of different devices.
2. Related Art
Bipolar semiconductor device technologies typically require multiple devices having different breakdown voltages on the same wafer for various circuit applications. Devices with different breakdown voltages have different performances due to the change in the collector transit time of the device that is obtained by varying the collector impurity profile between devices, which also modulates the breakdown voltage.
Multiple breakdown voltage devices are typically obtained by using a different mask and implant to tailor the collector impurity profile for each different breakdown voltage and performance device. This drives need for separate different masks and implants to tailor the collector profile for each different breakdown voltage and performance device. As a result of the additional mask/implant steps, bipolar technologies are expensive to implement. Accordingly, there is a need for bipolar technologies that provide lower cost implantation for generating these collector impurity profiles such that the cost of extra mask levels and implants are minimized. Some low cost alternative approaches include sharing the N-well mask/implant for the reach-through and collector of a device. However, these approaches are unsatisfactory because the thick resist needed to block the deep N-Well implants compromises the groundrules needed to minimize the size of the NPN device.
It is known to those skilled in the art of advanced complementary metal-oxide semiconductor (CMOS) device design and fabrication that N-Well-like implants may scatter out of the edge of the resist which blocks the implant in the field region. These scattered ions may disadvantageously dope the surface of the exposed silicon closest to the edge of the resist. This results in transistors having different threshold voltages depending on the transistor's proximity to the edge of the resist opening.
In view of the foregoing, there is a need in the art for a method of varying an impurity profile region in collectors of multiple devices on a single wafer to vary breakdown voltages using fewer masks and implants.